This invention relates to a method for the preparation of a halogenated aromatic amine, wherein a halogenated aromatic nitro compound is hydrogenated in liquid phase to give a corresponding halogenated aromatic amine.
Generally, it is well known in the art that when a halogenated aromatic nitro compound is hydrogenated, there occurs dehalogenation reaction as well as reduction of the nitro groups, so that qualities and yield of the resultant halogenated aromatic amine become lower. Furthermore, hydrogen chloride which is produced by the dehalogenation reaction greatly contributes to corrosion of the reactor. Accordingly, it is required that the dehalogenation reaction be suppressed to a minimum in the hydrogenation of the halogenated aromatic nitro compound. There have been heretofore known various methods for suppressing the dehalogenation reaction, including a method of using a metallic sulfide as a catalyst and a method of adding a dehalogenation inhibitor. In this connection, there are described methods of using sulfides of noble metals as a catalyst in French Pat. No. 1,417,236 and also by Harold Greenfield et al (Journal of Organic Chemistry, Vol. 32 Page 3670). However, the sulfide catalysts are disadvantageously lower in catalytic activity than noble metal catalysts, and higher in production cost since they require complicated preparation process.
Furthermore, there are known a variety of methods of conducting hydrogenation by the addition of a dehalogenation inhibitor, for example, a method of using a platinum-base catalyst together with magnesium oxide or hydroxide as a dehalogenation inhibitor (British Pat. Specification No. 859,251), a method of using a morpholine or a piperizine (U.S. Pat. No. 3,145,231), a method of using divalent nickel and trivalent chromium ions together with ammonia, a morpholine or a piperazine (U.S. Pat. No. 3,546,297), a method of using triphenylphosphite or tritollylphosphite (U.S. Pat. No. 3,474,144), etc.
Among these, the method of U.S. Pat. No. 3,546,297 has a disadvantage that though little dehalogenation reaction occurs, the preparation of the catalyst including nickel and chromium is very complicated, the loss of the ammonia during operation is large in amount due to its high volatility particularly where ammonia is used, so that it is difficult to handle ammonia quantitatively. Furthermore, any of methods of British Pat. Specification No. 859,251 and U.S. Pat. Nos. 3.145.231 and 3,474,144 is disadvantageous in that the effect of inhibiting dehalogenation is insufficient.
As for a method of using a catalyst other than of platinum, there is known a method of employing nickel as a catalyst and as an inhibitor an inorganic alkali or salt thereof such as magnesium hydroxide (U.S. Pat. Nos. 3,051,753 and 3,067,253) or a method of using a thiocyanate as an inhibitor (British Pat. Specification No. 1,191,610). However, the abovementioned methods are inferior in effects in inhibiting dehalogenation to the method employing a platinum catalyst.